Using this trick you will be able to lock your computer with a Usb.The computer will work only when the Usb is plugged in. Once the Usb is removed keyboard and mouse will automatically get disabled and your screen will get dark.Moreover your computer will get automatically locked. To unlock the computer you will need to insert the Usb again.
This trick will work on on all versions of windows including both windows 32 bit and 64 bit. For Trick to lock Your Computer With Usb Using Predator Follow this Steps: 
1. Download “predator” software.
 2. Predator will get launched automatically after completing installation if not you can run it fromStart/Programs/Predator 
3. Now Insert your Usb.You will get a message to define your new password.(This process will not format your pendrive and your pendrive data will not be affected by this at all)
 4. Click on Ok and Enter your your new password in next dialog box 
5. Check that the drive letter displayed under “USB key drive” actually matches your flash drive or choose the correct letter from the dropdown list 6. Finally click on Create Key button and then on OK button.